1. Field of the Invention:
The present invention relates to improved organopolysiloxane compositions curable into elastomeric state, even in a confined atmosphere, and, more especially, to improved organopolysiloxane compositions comprising an acyloxy crosslinking agent and a natural or synthetic hydrogel hardening agent.
2. Description of the Prior Art:
Compositions of the above general type, with optionally accelerated curing, and comprising acyloxy radicals, are known to this art. They are described, in particular, in British Patent No. GB-A-1,308,985, where the hardening agent is a zeolite containing water, and in European Patent Nos. EP-A-118,325 and EP-A-117,772, where the hardening agent is selected, respectively, from an alkali and alkaline earth metal hydroxide, or else from a mixture of water and an alkali or alkaline earth metal phosphate and/or polyphosphate.
These compositions are stable in storage only if they are maintained separately apart as 2 components (or in 2 packages). In fact, the hardening agent, packaged in one of the 2 components, contains water and can release and/or produce water during the mixing of the 2 components, which is generally followed by heating.
In addition, these compositions have a rather short crosslinking (or curing) time, on the order of a few minutes to 60 minutes. This crosslinking is, furthermore, independent of the moisture content of the surrounding atmosphere, since the water may be contributed solely by the crosslinking agent.
These compositions are therefore more particularly employed in fields of application where this property is particularly advantageous, such as, for example, the production of "in situ" seals in the motor-vehicle industry.
The above characteristics permit these compositions to be clearly distinguished from the single-component compositions containing acyloxy or ketiminoxy radicals, which are stable in storage and which cure merely on contact with atmospheric moisture (see, for example, FR-A-No. 1,193,271; US-A-No. 3,061,575; FR-A-No. 2,429,811; EP-A-No. 102,268; EP-A-No. 157,580 and DE-A-No. 3,524,452).
In the case of industrial applications involving the continuous deposition of seals, an automatic tool is generally used for depositing a two-component diorganopolysiloxane composition (with 2 components) which has a mixing head equipped with a deposition nozzle, the latter translating along the outline of the seals to be produced.
In order to avoid the composition setting solid in the mixing head, which is maintained at ambient temperature, during its operation or during stoppages on a continuous sealing line, it would be desirable to have access to a composition exhibiting, on the one hand, a slow crosslinking at high ambient temperature (longer than at least about ten minutes) after the two components have been mixed, namely, after mixing the cure accelerator and, on the other hand, a very fast crosslinking of the seal after it has been deposited.
Complete control of the crosslinking time at ambient temperature would make it possible to stop a continuous sealing line without the need for purging the mixing head. A fairly slow setting at ambient temperature would also be beneficial for obtaining a suitable adhesiveness of the seal to the substrates, because the composition would spread readily on these substrates with a sufficiently long contact time to ensure proper wetting of the surface, since the adhesiveness of the seals to the substrates is frequently an essential property of the seal.
It is also desirable to have available a composition which sets rapidly under the influence of a temperature increase which is as low as possible. In point of fact, a moderate temperature increase would make it possible to maintain high production rates on continuous industrial sealing lines.
In respect of the elastomer obtained, a composition of this type would also have to provide good mechanical properties, particularly when heated and in a confined atmosphere, with regard to the residual compression set (RCS).
The known compositions with optionally accelerated curing do not exhibit the combination of the desired properties referred to above. In fact, the compositions in which the hardening agent is zinc oxide and water, or else zeolites, do not exhibit satisfactory mechanical properties, especially when heated in a confined atmosphere.
The compositions described in European Patent Nos. EP-A-118,325 and EP-A-117,772 represent a very considerable progress with regard to the retention of mechanical properties when heated in a confined atmosphere, and have presented the possibility of employing these compositions for adhesive bonding or for sealing components travelling on industrial assembly lines, or in the case of which there are no available storage areas which would ensure their complete cure. However, these compositions can exhibit a setting time at ambient temperature which is too short and an adhesiveness to the substrate, in particular a metallic substrate, which may be inadequate.